While it is clear how to compare theoretical predictions of
a given model with observations, the reconstruction of
a phenomenological model from the data is a more subtle issue.
The basic problem is that in the CMB, we see the whole history
of the evolution in redshift projected onto the two dimensional
sky. The reconstruction of the evolutionary history of the universe
might thus seem an ill-posed problem.

Fortunately, one needs only to assume the very basic
properties of the cosmological model and the gravitational
instability picture before useful information may be extracted.
The simplest example is the combination of
the amplitude of the temperature fluctuations, which reflect
the conditions at horizon crossing, and large
scale structure today. Another example is the acoustic peaks in the
temperature which form a snapshot of conditions at last
scattering on scales below the horizon at that time.
In most models, the acoustic signature provides a wealth of
information on cosmological parameters and structure formation
([Hu & White] 1996). Unfortunately, it does not directly
tell us the behavior on the largest scales where important
causal distinctions between models lie. Furthermore it
may be absent in models with complex evolution on small scales
such as cosmological defect models.

Here we shall consider how polarization information aids
the reconstruction process by isolating the last scattering
surface on large scales, and separating scalar, vector and tensor
components. If and when these properties are determined, it will
become
possible to establish observationally the basic properties
of the cosmological model such as the nature of the initial fluctuations,
the mechanism of their generation, and the thermal history of the universe.